The invention relates to a method for the determination of the pressure in and/or of the through-flow through a pump with at least one magnetic bearing for the axial journaling of the pump rotor.
Pumps in which the pump rotor is magnetically journaled are nowadays already being used in a number of fields of application. In particularxe2x80x94but not exclusivelyxe2x80x94these are applications in which mechanical journallings of the rotor cannot be tolerated as a result of possible contamination of the medium to be forwarded, such is for example the case in blood pumps. Here magnetic journallings of the rotor are the rule.
Pumps with magnetic journallings of the rotor are for example known from DE-A-196 13 388 (U.S. Pat. No. 5,735,357) or from U.S. Pat. No. 4,781,525. Pumps can be described in more detail through several characteristic parameters. Two important characteristic parameters of this kind are the pressure in a pump and the through-flow through a pump. These parameters can be determined in principle with corresponding devices for the pressure measurement and the through-flow measurement respectively, but this typically has the result that corresponding sensors must be provided in the pump. Precisely in blood pumps, however, it is desirable if these parameters can be determined without sensors.
For this it is proposed in the already named U.S. Pat. No. 4,781,525 to determine the through-flow in such a manner that the torque by means of which the rotor is driven is determined (via the current by means of which the rotor is driven) at a constant viscosity of the liquid to be forwarded and at a predetermined speed of rotation of the rotor. At constant viscosity this torque is linearly related to the through-flow according to U.S. Pat. No. 4,781,525 (column 1, lines 57 ff., column 3, lines 50 ff.). The through-flow is determined with the help of various tables (see FIG. 2 of U.S. Pat. No. 4,781,525), each of which represents the linear relationship between the torque and the through-flow for a constant viscosity. It is noted that this linear relationship firstly holds only very approximately and secondly is valid only for limited through-flow amounts. Considered realistically, the relationship between the torque and the through-flow (at a predetermined speed of rotation) is not linear, however; this holds all the more above a certain through-flow amount.
It is an object of the invention to be able to determine the pressure in and/or of the through-flow through a pump of this kind without sensors (thus without special sensors for pressure and through-flow) and in this to obtain values for the pressure and the through-flow which are as realistic as possible without the need to measure the pressure and the through-flow directly. The present invention is dedicated to this object.
In accordance with the invention a method is proposed in which the pump used has at least one magnetic bearing for the axial journalling of the pump rotor, with axial forces acting on the rotor during the operation of the pump. For the determination of the pressure and/or of the through-flow, the axial forces which act on the rotor are utilized in the method in accordance with the invention. The axial forces which act on the rotor can be simply determined in various manners.
The axial forces (axial thrust) which act on the pump rotor are namely dependent on the pressure difference between the inlet side of the pump (e.g. centrifugal pump, diagonal pump, axial pump) and its outlet side. Therefore the axial forces (axial thrust) which act on the pump rotor can be used in order to determine this pressure differencexe2x80x94this pressure difference is generally designated as the xe2x80x9cpressurexe2x80x9d in the pump.
In a method variant the axial magnetic bearing of the pump rotor is designed as an active journalling. For the determination of the axial forces which act on the rotor then the bearing current which is required in each case for the axial journalling of the rotor is utilized.
In another method variant the axial magnetic bearing of the pump rotor is designed as a passive journalling. For the determination of the axial forces which act on the rotor the axial deflection of the rotor is then utilized.
Depending on the type of the axial magnetic journalling (active or passive respectively) different journalling currents which are required for the axial journalling of the rotor or different axial deflections of the rotor respectively at different speeds of rotation as well as the respective pressures resulting therefrom can in each case be stored in a look-up table. This look-up table is prepared prior to the practical use of the pump. During the operation of the pump the respective journalling current or the respective axial deflection of the rotor respectively and its speed of rotation are then determined, and with the help of this look-up table the pressure resulting therefrom is then determined.
Since larger item numbers of a pump type are regularly manufactured, the look-up table need be generated only a single time, insofar as it is ensured that the manufactured pumps remain within predetermined tolerances. Of course it is also possible to generate and store a look-up table of this kind for each individual pump, through which however the cost and complexity is increased; but the precision is also increased in return.
As an alternative to the look-up table the pressure in the pump can be determinedxe2x80x94depending on the kind of the axial journalling (active or passive respectively)xe2x80x94with the help of a polynomial from the respective journalling current which is required for the axial journalling of the rotor or from the respective axial deflection of the rotor respectively on the one hand as well as from the respective speed of rotation on the other hand. A polynomial of this kind likewise permits the pressure in the pump to be determined with a sufficient precision.
The higher the precision requirements are in this situation, the higher will be the order of the polynomial as well, and the greater will be the computational effort. In this the coefficients of the polynomial are likewise determined prior to the practical use of the pump. Here as well it holds that the coefficients for a given pump type need only be determined a single time when predetermined manufacturing tolerances are observed. They can also be determined individually for each individual pump, however, which increases the precision, but also increases the cost and complexity.
In a further development of the above-named method variant the through-flow through the pump is determined from the already established pressure and the associated speed of rotation. The determination of the through-flow is done via the pressure-through-flow characteristics of the pump (also called xe2x80x9crestrictor curvesxe2x80x9d).
These pressure-through-flow characteristics of the pump can again be stored in an electronic look-up table which has been generated prior to the practical use of the pump, either once for each pump type or for each pump separately. During the operation of the pump the through-flow through the pump is then determined with the help of this look-up table from the already established pressure.
Alternatively to the look-up table, the pressure-through-flow characteristics can be approximated by a polynomial so that during the operation of the pump the through-flow through the pump is then determined from the already established pressure with the help of this polynomial. The above considerations hold for the generation of the polynomial.
In a variant of the above-named method a pump with sensors for the determination of the direction of the magnetic field in the gap between the rotor and the stator is used. These sensors (typically Hall sensors) are also used in this method variant for the determination of the axial deflection of the rotor.
In this method variant the through-flow through the pump can be determined in that the torque which drives the rotor and the speed of rotation of the rotor are determined. For the determination of the torque the magnetic drive flux in the gap between the rotor and the stator and the drive current for the rotor are determined on the one hand and the axial deflection of the rotor is determined on the other hand. From the drive current and the axial deflection of the rotor the driving torque is then determined so that finally the through-flow through the pump is determined from the thus established torque and the established speed of rotation.
For this, different drive currents at different axial deflections of the rotor and the respective torque resulting therefrom can be stored in an electronic look-up table which has been generated and stored prior to the practical use of the pump. During the operation of the pump the torque is first determined from the respective axial deflection of the rotor and the respective axial drive current with the help of this look-up table. Then the through-flow through the pump is determined from the thus established torque and the established speed of rotation.
This determination of the through-flow of the pump from the established torque and the speed of rotation of the pump rotor is done via the so-called power-flux characteristics. These power-flux characteristics are basically non-linear (in contrast to the linear assumption for the relationship between the flux and the torque in U.S. Pat. No. 4,781,525, which is named above) and can be stored in the form of an electronic look-up table; they can however also be approximated by a polynomial.
In a further development of the above-named method variant the pressure in the pump is determined from the already established through-flow and the associated speed of rotation via the pressure-through-flow characteristics (xe2x80x9crestrictor curvesxe2x80x9d).
These pressure-through-flow characteristics (xe2x80x9crestrictor curvesxe2x80x9d) of the pump can, as already mentioned above, be stored in an electronic look-up table so that during the operation of the pump the pressure in the pump is then determined from the already established through-flow with the help of this look-up table.
Alternatively, the pressure-through-flow characteristics (xe2x80x9crestrictor curvesxe2x80x9d) can be approximated by a polynomial, as has likewise been mentioned above, so that during the operation of the pump the pressure in the pump is determined from the already established through-flow with the help of this polynomial.
Those method variants described above in which the through-flow is determined started from a given viscosity. If it is now desired to increase the precision, then the viscosity of the fluid in the pump can be determined for the purpose of the determination of the through-flow from the previously established pressure or for the purpose of the determination of the through-flow from the torque and the speed of rotation respectively. For this the following variants are available.
In a first variant, for the determination of the viscosity of the liquid, the pressure which is established with the help of the axial forces which act on the rotor is compared with the pressure which results from the determination with the help of the established torque and the established speed of rotation. For this the pressure must however also be determined in both ways. The viscosity is then determined from the difference of these two pressures and from the associated speed of rotation.
In a second variant the liquid damping which acts on the rotor is established for the determination of the viscosity of the liquid, and the viscosity is then determined from the established liquid damping. This is possible because each magnetic bearing can be characterized by its stiffness and by its damping. Considered physically the viscosity is a measure for the liquid friction (which is proportional to the speed) and thus has the effect of a damping.
If the magnetically journalled rotor is now located in a liquid, the total damping of the systems is above all increased in comparison with the operation in air. Since the total damping of the system is substantially additively composed of the damping of the magnetic bearing and the liquid damping, the liquid damping forms a measure for the viscosity of the liquid. The liquid damping can now be determined either directly or indirectly via the system damping (which is of course additively composed of the known damping of the magnetic bearing and of the liquid damping). For this a plurality of variants are available.
In a first variant the liquid damping is determined with the help of the shifting of the eigen-frequencies (rigid body oscillations, bending deflection oscillations) of the rotor.
In a second variant the liquid damping is determined in that the rotor is operated at an eigen-frequency and the deflection of the rotor thereby produced is determined. With the help of the deflection of the rotor the liquid damping is then determined.
In a third variant the liquid damping is determined in that the speed of rotation of the rotor is varied with different frequencies and the variation of the axial journalling forces or of the axial deflection respectively which arises at the respective frequency of the variation of the speed of rotation is determined. The liquid damping is then determined from the variation of the axial journalling forces or of the axial deflection of the rotor respectively at different frequencies of the variation of the speed of rotation.
Finally in a fourth variant the liquid damping is determined in that the displacement of the stability boundary of the control circuit for the axial magnetic journalling of the rotor is determined. The liquid damping is then determined from this displacement of the stability boundary of the control circuit for the axial magnetic journalling of the rotor.
Once the viscosity has been determined, then, as already mentioned above, in those method variants in which the through-flow is determined from the previously established pressure (via the xe2x80x9crestrictor curvexe2x80x9d) or in which the through-flow is determined from the torque and the speed of rotation (via the power-flux characteristics) respectively, the established viscosity can be taken into account, through which the precision of the values for the through-flow or the pressure respectively is increased even further.